That way lies madness [LWN.net]

That way lies madness

Posted Apr 2, 2012 14:30 UTC (Mon) by Richard_J_Neill (subscriber, #23093)
In reply to: That way lies madness by intgr
Parent article: The x32 system call ABI

This is quite a clever trick. If I understand rightly, what Java is doing is giving up byte-addressability, in favour of more address space. I.e. you can't create a pointer to a byte/char any more; the smallest data-type then becomes an int, and strings have to contain 4*n bytes. Given that x86 accesses memory 32-bits at a time anyway, this is a fairly natural thing to do.

to post comments

That way lies madness

Posted Apr 3, 2012 20:49 UTC (Tue) by ibukanov (subscriber, #3942) [Link] (1 responses)

> Given that x86 accesses memory 32-bits at a time anyway,

On modern CPU memory is addressed internally by cache lines that are typically 16-32-64 bytes in size. On x86 the byte access is just as fast as 32-bit access. Moreover, misaligned access to 32-bit values is allowed and is not costly as long as the variable does not cross the cache line boundary.

That way lies madness

Posted May 21, 2012 15:08 UTC (Mon) by mikemol (guest, #83507) [Link]

For basic instructions, yes. Take a look at the SSE instructions; while there are unaligned and aligned versions for several, the aligned versions will carry better performance.